Thermal flowmeters of the type wherein fluid passes through a length of heated conduit and the temperature of one or more points along the conduit is detected to determine flow rate are well known. Examples of such flowmeters are those disclosed in U.S. Pat. Nos. 3,181,357 and 3,229,522 granted to James M. Benson on May 4, 1965 and Jan. 18, 1966, respectively.
In the aforesaid patents, the conduit is directly heated by induced currents. Other types of heated conduit flowmeters employ an insulated heater wire wrapped around the conduit to impart heat thereto.
With respect to known arrangements, stable long term heating of the conduit is achieved. However, the curve plotting temperature gradients along the conduit tends to be flat and AC noise arising from the heating operation is picked up by the thermocouple sensors. This latter deficiency is particularly found in the arrangements of the type disclosed in U.S. Pat. Nos. 3,181,357 and 3,229,522, inasmuch as the heating current at the respective sensors is out of phase. Both the flatness of the temperature gradients and the generated AC noise adversely affect the sensitivity of the thermocouple outputs thereby limiting the accuracy of the flow detection.
In the case of systems using an insulated wire heater to heat the conduit, even though the curve of temperature gradients is less flat than in the case of induced current heating, long term stability is not available due to changes in the characteristics of the insulating material.